Large tanks are used to store petroleum, crude oil and various types of partly refined oil. While the oil is being stored, higher or lighter fractions of the oil can separate from heavier fractions. As more and more of the heavier fractions separates out of the oil, the heavier fractions become more and more viscous and can form a “sludge” on the bottom of the tank. This sludge will tend to stay in the tank as the oil in the tank is removed and new oil is stored in the tank. As oil is continually removed from the tank and new oil stored in the tank, the sludge can continue to build up and can reduce the available storage space in the tanks because more and more of the storage capacity of the tank is being used up by this sludge that is staying in the tank.
There are a number of methods presently used to remove this sludge, including removing as much oil as possible from the tank and simply trying to manually excavate the sludge from the tank. However, these tanks are usually quite large; often holding thousands of tons of oil product and can measure 40 feet high or more with diameters of 50 feet to 200 feet or even more. Therefore, manually removing the sludge can be labor intensive, take quite a long time and be quite dangerous.
Another method that is often used to try and remove the sludge from these tanks is to use a water jet to break up the sludge and mix it into an emulsion with water or other liquids so that the emulsified sludge can then be drained from the tank. Often these large tanks will have one or more manway covers provided at or near the ground level. These manway covers are commonly circular access doors that bolt onto the outside of the tank. By undoing the bolts and pulling off the manway cover, a person can gain access to the inside of the tank Often the water jet is provided installed in a manway cover sized plate that bolts into the opening where the manway has been removed. This water jet is rotatable using a ball and socket joint. With the water jet installed in the manway cover it can be used to blast water into the tank to try and mix up the sludge so it can be drained. A person can move the hose attached to the water jet which in turn will rotate the nozzle with the ball and socket to re-direct the direction of the water jet.
However, the problem with this method is that the manway covers are placed quite low in the tank (typically just above the ground level so that they are easily accessible by a person standing on the ground) often the sludge can build up to a level higher than the manway cover and cause a spill on the ground when the manway cover is removed. Even if a spill is not caused right away, the sludge is often not of a consistent height in the bottom of the tank and instead form ridges and valleys inside the tank. When the manway cover is removed, sludge might not spill out of the manway immediately, however, once it is disturbed during the installation of the water jet, a spill could occur at any time during installation.